Capacitors for use in electronic instruments are demanded to have a small size and a large capacitance. As one example of such capacitors, a capacitor having an etched aluminum foil for one part electrode is known. In this capacitor, the electrode is etched from the surface toward the inside to a predetermined thickness whereby the actual surface area is enlarged. As the surface area is larger, the capacitance can be larger. Therefore, etching conditions, etching method, etching agent and other related factors have been studied with an attempt to enlarge the surface area of electrode. With respect to the etching method, a chemical etching method and an electrical etching method were usually combined to reduce the etching pore size or increase the etching depth to thereby enlarge the surface area. However, there was a limit in enlarging the surface area while keeping various strengths of the electrode. Studies have also been made to more increase the thickness of electrode than usual to thereby attain the enlargement of surface area and the maintenance of high strengths, however, this contradicts the downsizing of capacitors and therefore could not be a substantial improvement.
On the other hand, as a capacitor having a small size and a large capacitance, a capacitor provided with a sintered body of tantalum for one part electrode is known. This capacitor is made by molding a tantalum powder and then sintering the thus-obtained molding, and, in the capacitor, the surface area of pores among powders inside the sintered body contributes to the capacitance of capacitor. However, in the case of a relatively large sintered body, it is difficult to impregnate another part electrode (usually, a solid compound) into the vicinity of the center of sintered body. AS a result, the appearance capacitance is disadvantageously small for the surface area. Furthermore, the distance from the outer surface to the center of the sintered body is long, and therefore, if the another part electrode is the same, a capacitor using the sintered body for the electrode is inferior in the high frequency performance as compared with the capacitor using the above-described etched aluminum foil for the electrode.
Despite these problems, in a capacitor using the tantalum sintered body for one part electrode, an oxide of tantalum can be used as a dielectric material, and the dielectric constant of the tantalum oxide is about three times as large as the dielectric constant of the aluminum oxide. Therefore, tantalum is an attractive material for enlarging the capacitance, and many studies have been made thereon so as to solve the above-described problems.
One example of such studies is an attempt to manufacture an etched foil of tantalum as an electrode material. However, desired etching of the foil of an earth-acid metal itself including tantalum is difficult or even impossible. For example, tantalum is soluble in hydrofluoric acid and therefore, a method of electrolytically oxidizing tantalum in hydrofluoric acid to manufacture an etched foil was employed. However, in practice, only edge parts of tantalum foil dissolved and etching of the entire foil surface was impossible. Studies were once made to incorporate additives to a tantalum starting material to thereby improve the etching property, however, this could not be a remarkable technical improvement. For example, a proposal has been made in Japanese Unexamined Patent Publication No. S46-7251 (the term “Japanese Unexamined Patent Publication” as used herein is hereinafter abbreviated to “JP-A”), wherein a tantalum foil was obtained from a mixture of tantalum with a vanadium compound, or a mixture of tantalum with a tantalum-vanadium alloy, but, the etching property was not improved.
Niobium belongs to the same group as tantalum. Niobium oxide has higher dielectric constant than tantalum oxide, and therefore, it is considered that by manufacturing an etched foil of niobium, a capacitor having a higher capacitance can be produced. However, this has not been practiced up to the present.